1000 FOODS AND FOOD ADULTERANTS. 



JOHANSON, E. A modification of Davy's method, employing 5 to 8 drops of a solu- 

 tion of chromic sulphate (one part in twenty-five parts water) per 100 cc of 

 the glue solution. Chem. News, 50, 176. 



JACKSON, E. This method employs lead carbonate as a precipitant of tannin. The 

 percentage of tannin is calculated from the density of the solution before 

 and after the precipitation. An error is possible from the partial solution of 

 the lead by a free organic acid. Chem. News, 50, 179; Zeitsch. f. anal. 

 Chem., 24, 271. 



JEAN, F. A volumetric method based upon the fact that tannic and gallic acids 

 combine with iodine, in the presence of sodium carbonate, in definite propor- 

 tions. Filter paper covered with starch powder is used in testing for excess 

 of iodine. The presence of gallic acid necessitates a second titration after 

 the removal of the tannin by an appropriate precipitant. Ber. d. cheui. 

 Ges., 9, 730; Zeitsch. f. anal. Chem., 16, 123. 



JEAN, F. A colorimetric method, in which a tannin solution of known strength (1 

 per cent) is added to a ferric chloride solution containing free hydrochloric 

 acid, until a piece of white paper placed under the beaker on black cloth is 

 no longer visible. A similar titration is made with a solution of the sample 

 to be assayed. Two titrations are necessary, one before and one after the 

 removal of the tannin by gelatin. Bui. Soc. chim. de Paris., 44, 183; 

 Zeitsch. f. anal. Chem., 28, 108. 



KATREINER. This chemist objects to the methods of Carpene and Barbieri, and 

 states that the probabilities of error are great. Zeitsch. f. anal. Chem., 18, 

 112. 



LOWE. In this method the infusion is evaporated with a few drops of acetic acid, 



and residue is then exhausted with alcohol, which dissolves the tannin only. 



The tannin is then estimated in the alcoholic solution. Jsb. d. Chem., 



18, 743. 



LEHMAN. Titration with a glue solution containing ammonium chloride. Jahres- 



ber. d. Chem., 1881, 1206; Russ. Zeit. Pharm., 1881, 321. 



L.OWENTHAL, J. This method is one of the most important yet devised for tannin. 

 It is given in considerable detail, since a modification of this method is usu- 

 ally employed in modern practice. See method as modified by Procter, 

 Conncler, and Schroeder, page 890, this report. 



The method devised by Lowenthal is based upon the oxidation of tannin by per- 

 manganate of potassium. Four solutions are employed: (a) permanganate of potas- 

 sium, standard solution; (J>) 6 grams indigo, with 50 cc concentrated sulphuric acid 

 per liter; (c) 25 grams white glue, dissolved in water and the solution saturated 

 with sodium chloride, then made up to 1 liter; (d) a saturated solution of sodium 

 chloride, with 25 cc sulphuric acid per liter. The sample is extracted in boiling 

 water, the solution is cooled and made up to 1 liter. (1) 10 cc of this extract are 

 diluted to 800 or 1,000 cc, 25 cc indigo solution added, and a titration is made with 

 the permanganate. (2) 25 cc indigo solution are diluted to 800 or 1,000 cc. and 

 titrated with permanganate. This burette reading is subtracted from that under (1) 

 to obtain the net reading, i. e., the permanganate required for the tannin and gallic 

 acid and other oxidizable matter in 10 cc of the extract. (3) 100 cc of the extract 

 are now treated with 50 cc glue solution, and, after shaking, 100 cc salt solution 

 are added. After several hours the solution is filtered. To 50 cc of the filtrate 25 

 cc indigo are added and another titration is made. I'mler these conditions, again 

 determine the permanganate required for the indigo and deduct tlie burette, reading 

 ftg before. In the titration under (3) a volume of the solution equal to 20 cc of the 

 extract was taken; hence divide the net burette rending by 2. This yives the per- 

 manganate required to oxidi/e the substances other than tannin in 10 cc of the ex- 

 tract. Deducting this last amount <if penn:mg:m:ite from that required for all the 

 oxidizable matter in the extract, and the remainder is that required for the tannin 

 in 10 cc extract. 



